Various fuel systems may be used to deliver a desired amount of fuel to an engine for combustion. One type of fuel system includes a port fuel injector and a direct injector for each engine cylinder. The port injectors may be operated during engine starting to improve fuel vaporization and reduce engine emissions. The direct injectors may be operated during higher load conditions to improve engine performance. In addition, both port injectors and direct injectors may be operated under some conditions to leverage advantages of both types of fuel delivery or differing fuels.
Direct injection fuel systems may include a high pressure fuel pump upstream of a fuel rail to raise a pressure of the fuel delivered to the engine cylinders through the direct injectors. However, when the high pressure fuel pump is turned off, such as when no direct injection of fuel is requested, pump durability may be affected. Specifically, the lubrication and cooling of the pump may be reduced while the high pressure pump is not operated, thereby leading to pump degradation.
Various approaches have been developed to reduce high pressure pump degradation. In one approach, as shown by Faix et al. in U.S. Pat. No. 6,230,688, a constant fuel lubrication flow quantity is branched off from the delivery flow of a low pressure pump coupled to a fuel tank, and delivered to a high pressure pump. However, the inventors herein have identified a potential issue with such an approach. The flow diversion and maintenance of a lubrication flow requires additional components which may add to operational costs.
Thus, in one example, the above issue may be at least partly addressed by a method of operating an engine fuel system including a first lower pressure fuel pump coupled upstream of a second higher pressure, positive-displacement fuel pump. In one embodiment, the method comprises, when second pump operation is not requested, operating the first pump to maintain a fuel rail pressure in the fuel rail while supplying fuel to an engine cylinder through an injector, and adjusting operation of the second pump to maintain an elevated pressure in a pump chamber of the second pump below the fuel rail pressure. In this way, lubrication and cooling of the higher pressure pump may be achieved using the available pump components, and without requiring additional components. As such, the above approach may be used with mechanical adjustments and additional components, such as those discussed in the above art, but may also be used in place of them.
In one example, an engine may include a first lower pressure pump configured to supply fuel from a fuel tank to an engine cylinder via a port injector, and a second higher pressure pump configured to receive fuel from the first pump, and supply fuel to the engine cylinder via a direct injector. The second pump may be a positive-displacement pump including a pumping chamber of varying volume. An inlet of the pumping chamber may be coupled to the lower pressure pump via an inlet check valve, and an outlet of the pumping chamber may be coupled to a fuel rail via an outlet check valve. During conditions when second pump operation is requested, such as when a fuel level in the fuel tank is higher than a threshold and direct injection of the fuel is requested, operation of the first and second pump may be adjusted and coordinated to maintain a desired fuel rail pressure while supplying fuel to the engine through the direct injector. In comparison, when second pump operation is not requested, such as when a fuel level in the fuel tank is lower than the threshold and/or direct injection of the fuel is not requested, operation of the first pump may be adjusted to maintain a desired fuel rail pressure to supply fuel to the engine via the port injector, while also enabling the second pump to be sufficiently filled with liquid fuel. Further, a stroke amount of the second pump may be adjusted (e.g., via pump duty cycle adjustments) to maintain a pressure in the pumping chamber of the second pump at or just below the fuel rail pressure. Specifically, the stroke amount may be adjusted so that a pressure in the pumping chamber (that is, on top of the pump piston), is large enough to drive fuel through the piston bore interface (to ensure lubrication), without affecting the fuel rail pressure or cylinder fuel injection. By maintaining a pressure in the pumping chamber (herein also referred to as the pumping chamber outlet pressure) of the second pump just below the fuel rail pressure, without allowing fuel to flow out of the outlet of the second pump into the fuel rail, the second pump may be kept lubricated, thereby reducing pump degradation.
As such, the outlet check valve may prevent fuel flow from the outlet of the second pump into the fuel rail as long as the pressure in the pumping chamber is maintained at or below the fuel rail pressure. To confirm that the pumping chamber outlet pressure is high enough to provide the desired level of lubrication, a stroke amount of the second pump may be occasionally pulsed, or intermittently increased, to allow a small amount of fuel to flow from the pumping chamber of the second pump into the fuel rail. If the pumping chamber outlet pressure is sufficiently high (that is, at the desired lubrication level), the adjusted stroke amount and related fuel flow into the fuel rail causes a corresponding increase in the fuel rail pressure. In response to the pulse in the fuel rail pressure, the stroke amount of the second pump is immediately decreased to a stroke amount that does not affect the fuel rail pressure.
In this way, during conditions when high pressure pump operation is not requested, by maintaining an elevated fuel pressure in the pumping chamber of the high pressure pump, without flowing fuel out of the pumping chamber into a fuel rail, fuel may be driven through the piston-bore interface of the pump to keep the pump sufficiently lubricated. By maintaining high pressure pump lubrication via adjustments to a stroke amount and duty cycle of the positive-displacement pump, pump lubrication may be achieved without necessitating additional components for flow diversion and flow control, although these may be included if desired. By reducing non-use of the high pressure pump, and maintaining high pressure pump lubrication and cooling, pump degradation may be reduced.